ENGINE
M6040, M7040, WSM
1-M8
4. COOLING SYSTEM
[1] THERMOSTAT
Conventional thermostatically-controlled valves
(outlet water temperature control type) open against the
flow of coolant. In this design, the pressure (steam
pressure + water pump's discharge pressure) affects the
open/close performance of such valve. In other words,
the valve may be delayed in opening at a preset opening
temperature opening suddenly, above the preset
temperature. This is called the overshoot phenomenon.
The overshoot problem invites the undershoot
phenomenon too. Too much water cooled by the radiator
flows through the water passage, which suddenly closes
the valve below the thermostat's preset valve closing
temperature.
A repeated cycle of such overshoot and undershoot
phenomena is called the water temperature hunting.
This hunting problem may adversely affect the cooling
system parts, and also the engine and its related
components.
To cope with this trouble, the 07 series engine is
equipped with the flow control thermostat. The valve has
a notch to control the coolant flowrate smoothly in small
steps.
9Y1210143ENM0013US0
(1) Coolant Temperature
(2) Time
(3) Overshoot
(4) Notch
(A) Valve Lift Versus Flowrate
(B) Flowrate
(C) At Short Valve Lift
(D) At Medium Valve Lift
(E) At High Valve Lift
(F) Valve Lift
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